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 Rectifiers: Electronic Building Blocks

Virtually all electronic circuits require DC current. Since the power coming out of our walls is AC, this makes rectifiers, which derive DC power from AC power, essential in everyday applications. Rectifiers have as many uses as they have types. This article will try and explain the basics of a rectifier and some common terms associated with them. In order to understand how a rectifier works, one must first understand how a diode works.

Diodes Explained
Certain elements are normally insulators, but we can turn them into conductors with a chemical process called doping. We call these materials semiconductors and silicon and germanium are two of the best known examples. Silicon is normally an insulator, but if you add a few atoms of the element antimony, you effectively sprinkle in some extra electrons and give it the power to conduct electricity. Silicon altered in this way is called n-type (negative-type) because extra electrons can carry negative electric charge through it.

In the same way, if you add atoms of boron, you effectively take away electrons from the silicon and leave behind "holes" where electrons should be. This type of silicon is called p-type (positive type) because the holes can move around and carry positive electric charge.

Basically, when the two are sandwiched together, a barrier forms, called a P-N junction and surrounding it is what’s called a depletion zone. An article at pbs.org explains it simply that one side of the semiconductor boundary is like mud, one like water. If you try to get electricity to move from the mud side to the water side, there's no problem. The electrons just jump across the boundary, forming a current. But try to make electricity go the other way and nothing will happen. Electrons that didn't have to work hard to travel around the water side just don't have enough energy to make it into the mud side. (In real life, there are always a few electrons that can trickle in the wrong direction, but not enough to make a big difference.)

Once voltage is applied in the correct direction (forward bias) across the diode, the P-N junction shrinks and electrons can travel from one side to the other. Voltage applied in the opposite direction (reverse bias) causes the depletion zone to expand and prevents current from traveling. But, just like lightning through air (normally an insulator), enough voltage can break through the barrier – a condition known as breakdown. Zener diodes are designed to take advantage of this by acting almost like a flood gate. The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.

Rectifiers
Rectifiers are mostly used to convert AC to DC although they can be used for temperature detection and in radios. Diodes can be connected in several ways to make a rectifier to convert AC to DC. Although the current runs in one direction, it pulsates, so many times the current is further modified to make the current more smooth like a battery.

“In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three phase supply.”
Rectifier half-wave rectification http://en.wikipedia.org/wiki/Rectifier

A full-wave rectifier uses both positive and negative halves of the wave. This can be done using a center-tap rectifier used more commonly in low power applications, or in more common higher power applications a bridge rectifier is used.

Bridge rectifiers
A bridge rectifier contains four diodes required arranged to convert AC (positive or negative) to DC. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labeled “+” and -, the two AC inputs are labeled “~”.
Bridge rectifiers diagram http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
Bridge rectifiers photos http://www.kpsec.freeuk.com/powersup.htm
The main advantage of bridge rectifiers over conventional full-wave rectifiers is that with a given transformer, the bridge rectifiers produce a voltage output that is nearly twice that of conventional full-wave circuit. Another advantage of bridge rectifiers is the low ratio of peak inverse voltage to average output voltage.

Bridge rectifiers are available for more than just single phase circuits. In fact, the more phases in the circuit, the “smoother” the DC output is because the pulses are closer together due to the overlapping of the waves. “In any case of rectification -- single-phase or polyphase -- the amount of AC voltage mixed with the rectifier's DC output is called ripple voltage. In most cases, since “pure” DC is the desired goal, ripple voltage is undesirable. If the power levels are not too great, filtering networks may be employed to reduce the amount of ripple in the output voltage.”[http://www.allaboutcircuits.com/vol_3/chpt_3/4.html]


**Specifications subject to changes**

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